This invention relates to a photocoupler device which comprises a light-emitting element and a light-receiving element which converts a light, issued from said light-emitting element, into an electric signal.
Reference is now made to the conventional photothyristor coupler whose cross section is indicated in FIG. 1. The photocoupler comprises a light-emitting element 2 mounted on a lead frame 1.sub.1 and a photothyristor element 3 provided on a lead frame 1.sub.2. The elements 2 and 3 are facing each other. The coupler also comprises a light-permeable silicone resin layer 4, in a space defined between said two elements, to be used as a photocoupling route. The whole mass is molded by a layer 5 of epoxy resin. With the above-mentioned conventional photothyristor coupler, the object of ensuring the efficient photocoupling of a light issued from the light-emitting element 2 to a photothyristor element 3 is sometimes attained by mixing the epoxy resin with a suitable material to elevate its photoreflectivity, or, by applying a material of high photoreflectivity to an interface between the light-permeable silicone resin layer 4 and epoxy resin layer 5.
FIGS. 2 and 3 jointly represent a photothyristor element used with the conventional photothyristor coupler. FIG. 2 shows the cross section of said photothyristor. FIG. 3 is the pattern of an assembly of an electrode and semiconductor region as viewed from the major surface of said conventional photothyristor coupler. The photothyristor element of FIG. 3 comprises a semiconductor substrate comprising an N conductivity type base region 6, a P.sup.+ conductivity type anode region 7 and P conductivity type base region 8 both mounted on the major surface of said N conductivity type base region 6, and an N.sup.+ conductivity type cathode region 10 formed in said P conductivity type base region 8. A cathode electrode 11 is formed on the N.sup.+ type cathode region 10. A gate electrode 12 is mounted on the P conductivity type base region 8. An anode electrode 13 is deposited on the P.sup.+ conductivity type anode region 7. Referring to FIG. 2, reference numeral 9 denotes a P conductivity type isolation region. Reference numeral 14 represents a passivation film prepared from, for example, an oxide film. The thyristor element is fabricated by the planar element-forming process.
While impressed with a bias voltage in the forward direction, the conventional thyristor element is rendered conducting upon receipt of a light. In this case, it is to be noted that all beams of light entering the thyristor element do not render the thyristor conducting. Namely, only the light entering the hatched region 15 of FIG. 3 (observation is directed toward the major surface of the thyristor) acts effectively. The hatched region comprises the whole N.sup.+ conductivity type cathode region 10, the whole P conductivity type base region 8 and that portion of the N conductivity type base region 6 which is defined by the PN junction J1 between the N conductivity type base region 6 and P conductivity type base region 8 and the carrier diffusion length L in the N conductivity type base region 6. The hatched region will be referred to as "an effective light-receiving region".) The light which is impinged on the other regions, particularly the isolation region 6, takes essentially no part in rendering the thyristor element conducting.